A new approach for defining the nautical density using the flow pattern critical conditions of fluid mud around the walls of the ships is proposed based on the similarity between the form of shear flow as ships sail in the fluid mud layer and the gravity flow of fluid mud on the gentle slope. The new methodology can avoid the uncertainty of current ones. Relevant calculation method is provided. It is shown that the nautical density is related to the cruise speed and the thickness of the fluid mud layer. Under the general cruise speed and 0.5~1.5 m thick layer condition, the nautical density in the channel of the Lianyun port is about 1.25~1.35 g/cm3.